Coherent structures in the near-field of swirling turbulent jets: A tomographic PIV study

https://doi.org/10.1016/j.ijheatfluidflow.2017.12.009Get rights and content

Highlights

  • Dynamics of 3D velocity and pressure field in turbulent swirling jets was investigated by the time-resolved tomographic PIV technique.

  • Large-scale flow organization was studied for a low-swirl jet without central recirculation zone and high-swirl jet with bubble-type vortex breakdown and central flow recirculation.

  • For high-swirl jet, the first azimuthal mode corresponded to the entire flow precession, whereas for the low-swirl jet it was related to a weak intermittent precession of the vortex core.

  • Intensive axisymmetric coherent mode was found to persist even for the high-swirl jet with strong flow precession.

Abstract

The present paper reports on high-speed tomographic particle image velocimetry measurements of large-scale coherent structures in the near field of swirling turbulent jets. Three flow cases are considered: a jet without superimposed swirl; a jet with low swirl; and a high-swirl jet with bubble-type vortex breakdown and a central recirculation zone. Local pressure fluctuations and their correlations with velocity were evaluated based on the Poisson equation and an effective viscosity model. Spatial Fourier transform and proper orthogonal decomposition were applied to evaluate the energies of different azimuthal modes for different cross-sections of the jet and to extract coherent structures. Toroidal vortices were observed in the mixing layer of the non-swirling and low-swirl jets. In the latter case, the vortices broke up earlier due to the swirl effect and formed longitudinal vortex filaments in the outer mixing layer of the jet. Deviation of the jet centreline from the axis of nozzle symmetry was detected for both non-swirling and low-swirl jets. In the latter case, this deviation was attributed to the intermittent vortex core precession. The amplitude of the axisymmetric mode increased downstream of the non-swirling and low-swirl jets, with development of the ring-like vortices. For the low-swirl jet, this increase was also associated with intensive velocity and pressure fluctuations along the jet axis. Although the high-swirl jet was more turbulent, a long helical vortex could be distinguished from other smaller eddies in the outer mixing layer. The flow dynamics was associated with a strong flow precession around the central recirculation zone. The first azimuthal mode had the largest amplitude until two nozzle diameters downstream and contained a rotating coherent structure. The second most intensive mode was related to the opposite axisymmetric oscillations of the axial velocity of the annular jet and reverse flow in the central recirculation zone.

Introduction

Swirling jet flows occur in a number of technical devices, e.g., combustors. A deeper understanding of the structure and dynamics of swirling jet flow is important for the development of more efficient heat exchangers, mixers, and burners and to avoid undesirable phenomena, such as thermos-acoustic resonance in combustion chambers and blow-off of lean flames (Syred and Beer, 1974, Paschereit et al., 1998, Lieuwen et al., 2001). In particular, the development and interactions of large-scale vortices in swirling jets, including break-up of the jet's vortex core and formation of secondary vortex structures, affect the transition to turbulence and mixing (Varaksin et al., 2010, Varaksin et al., 2012, Nolan, 2012). Thus, the phenomena of vortex core breakdown (see Leibovich, 1978, Billant et al., 1998, Lucca-Negro and O'Doherty, 2001, Oberleithner et al., 2011, Oberleithner et al., 2012) and vortex core precession (Syred and Beer, 1972, Syred et al., 1997, Syred, 2006) in swirling flows have been studied extensively.

For non-swirling jets, large-scale toroidal vortex structures promote heat and mass exchange between the jet and surrounding fluid, whereas smaller-scale longitudinal vortices are important for mixing and, in particular, stabilizing fuel-rich flames (Yule, 1978, Liepmann and Gharib, 1992, Demare and Baillot, 2001, Kozlov et al., 2011). The dynamics of large-scale vortices and their interactions have impacts on the acoustic noise emitted by jets (Crighton, 1975). Imposition of a swirl on the jet promotes helical instability modes (Alekseenko et al., 2007), which start to dominate over the axisymmetric mode, corresponding to the formation of ring-like vortices (Gallaire and Chomaz, 2003, Gallaire et al., 2004). Further increases in the swirl rate result in a destabilization of the jet's swirling vortex core and its eventual breakdown (Sarpkaya, 1979, Billant et al., 1998).

Spiral-type vortex breakdown is observed for low and moderate swirl rates. It occurs during widening of the rotating flow (e.g., in a pipe with extension or during the issue of a jet from a nozzle) and deceleration of velocity along the vortex core. The flow loses its axial symmetry, and the vortex core takes the shape of a spiral. Local reverse flows may also appear at the flow axis in an intermittent manner. Further increases in the swirl rate result in a bubble-type or cone-type vortex breakdown (Sarpkaya, 1979, Spall, 1996, Billant et al., 1998), with a permanent central reverse flow in the shape of a bubble or cone, respectively. The recirculation zones have axisymmetric shapes only for small Reynolds numbers.

As concluded by Ruith et al. (2003), after the central recirculation zone appears, it triggers helical instability of the swirling jet's core downstream. According to Liang and Maxworthy (2005) and Oberleithner et al. (2012), formation of the central reverse flow gives rise to a global self-oscillating mode, which generates a coherent structure consisting of large-scale helical vortices (Cala et al., 2006, Oberleithner et al., 2011, Alekseenko et al., 2012, Markovich et al., 2014). The properties of these coherent structures in high-swirl jets and flames have been studied extensively with the aid of the conditional averaging technique and proper orthogonal decomposition (POD, Sirovich, 1987, Holmes et al., 1996). Vortex structures in swirling turbulent jets without a central recirculation zone have been studied in less detail (Markovich et al., 2016).

High-speed tomographic particle image velocimetry (tomographic PIV, see Scarano, 2012), which is currently under intensive development, provides a deeper understanding of the structure and dynamics of unsteady turbulent flows based on detailed analysis of time-resolved 3D instantaneous velocity fields and all components of the velocity gradient tensor (Violato and Scarano, 2011). In addition, local pressure fluctuations (at the scales resolved by PIV) can be evaluated reliably based on the Navier–Stokes equation (see Liu and Katz, 2006, de Kat and van Oudheusden, 2012, Ghaemi et al., 2012, van Oudheusden, 2013).

The present paper reports on time-resolved tomographic PIV measurements of large-scale flow organization and dynamics in turbulent non-swirling, low- and high-swirl jets. This work complements previous research by Markovich et al. (2016), in which 3D helical coherent structures in swirling flows were extracted by performing POD after the Fourier transform over the azimuthal angle. The current paper considers the swirl effect on the amplitude of azimuthal modes for different 2D cross-sections of the velocity and pressure fields. The dynamics of the swirling jet for low- and high-swirl rates corresponds to two most intensive modes. One is the precession of the vortex core, which occurs in an intermittent manner for the former case and involves almost the entire flow in the latter case. The other mode is related to the flow oscillations in the axial direction, which for the low-swirl jet occur mainly in the vicinity of the jet axis.

Section snippets

Jet flow facility

The swirling jet flow was organized in a closed hydrodynamic circuit that included a water tank, pump, thermostat, flowmeter, and test section. Flowrate stabilization was provided via feedback from the flowmeter to the pump. A fixed water temperature of 30 ± 0.4°C was provided by the thermostat, which controlled heat exchange between the main circuit and an additional circuit with cooling water. The rectangular test section (200 × 600 × 200 mm3) was fabricated from Plexiglas (see Fig. 1a). A

Results and discussion

The time-averaged velocity data measured previously by Alekseenko et al. (2008) using stereoscopic 2D PIV are plotted in Fig. 2 (Re = 8900). The axial velocity reaches negative values for the case S = 1.0 due to the presence of a central recirculation zone. The nozzle-exit distribution for the low-swirl jet velocity features a peak at the jet axis, which is produced during contraction of the swirling flow inside the nozzle (Billant et al., 1998). Downstream, the axial velocity decreases due to

Conclusions

Time-resolved tomographic PIV measurements of large-scale flow organization and dynamics in turbulent non-swirling and low- and high-swirl jets are reported in this paper. The swirling jets correspond to two substantially different flow cases: a low-swirl jet, where no recirculation zone was present in the mean velocity field, and a high-swirl jet with pronounced vortex breakdown and a bubble-type central recirculation zone. A tomographic PIV system with an acquisition rate of 2 kHz was used

Acknowledgements

This work was partially supported by the Russian Science Foundation (grant No. 14-19-01685). Fruitful discussions with Dr. Rustam Mullyadzhanov and Prof. Kemal Hanjalic are kindly acknowledged.

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